There are many examples of radio communications systems in which data is communicated using Orthogonal Frequency Division Multiplexing (OFDM). Television systems which have been arranged to operate in accordance with Digital Video Broadcasting (DVB) standards for example, use OFDM for terrestrial and cable transmissions. OFDM can be generally described as providing K narrow band sub-carriers (where K is an integer) which are modulated in parallel, each sub-carrier communicating a modulated data symbol such as for example Quadrature Amplitude Modulated (QAM) symbol or Quaternary Phase-shift Keying (QPSK) symbol. The modulation of the sub-carriers is formed in the frequency domain and transformed into the time domain for transmission. Since the data symbols are communicated in parallel on the sub-carriers, the same modulated symbols may be communicated on each sub-carrier for an extended period. The sub-carriers are modulated in parallel contemporaneously, so that in combination the modulated carriers form an OFDM symbol. The OFDM symbol therefore comprises a plurality of sub-carriers each of which has been modulated contemporaneously with different modulation symbols. During transmission, a guard interval filled by a cyclic prefix of the OFDM symbol precedes each OFDM symbol. When present, the guard interval is dimensioned to absorb any echoes of the transmitted signal that may arise from multipath propagation.
It has been proposed for a television system known as the Advanced Television Systems Committee (ATSC) 3.0 in a publication entitled ATSC 3.0 Working Draft System Discovery and Signaling [1] to include a pre-amble in a transmitted television signal which is carrying broadcast digital television programs. The preamble includes a so called “boots strap” signal which is intended to provide a receiver with a part of the transmitted signal which it can have a greater likelihood of detecting and therefore can serve as a signal for initial detection. This is because broadcasters anticipate providing multiple services, within a broadcast signal in addition to just broadcast television. Such services may be time-multiplexed together within a single RF channel. There is therefore a need to provide an easily detectable signal segment (the bootstrap signal) that is transmitted as part of a pre-amble to multiplexed frames, so that a receiver can discover and identify what signals and services are available.
It has been proposed [1] to make the bootstrap signal have a fixed configuration, including sampling rate, signal bandwidth, subcarrier spacing, time-domain structure etc. known to all receiver devices and to carry information to enable processing and decoding the wireless service associated with a detected bootstrap. This new capability ensures that broadcast spectrum can be adapted to carry new services and/or waveforms that are preceded by a universal entry point provided by the bootstrap for public interest to continue to be served in the future.
The bootstrap has been designed to be a very robust signal and detectable even at very low signal to noise ratio levels. As a result of this robust encoding, individual signalling bits within the bootstrap are comparatively expensive in terms of the physical resources that they occupy for transmission. Hence, the bootstrap is generally intended to signal only the minimum amount of information required for system discovery and for initial decoding of the following signal. However in order to detect payload data transmitted as OFDM symbols it is necessary to communicate layer 1 (L1) signalling data indicating communications parameters which have been used to carry the payload data as OFDM symbols.